1. Field of the Invention
The present invention relates to a hot press forming, and in particular to improvement of a plate-shaped workpiece positioning structure that positions a plate-shaped workpiece relative to a press die.
2. Description of Related Art
A positioning structure that, when a heated plate-shaped workpiece is subjected to a hot press forming, positions the plate-shaped workpiece relative to a press die and that has both (a) a first positioning opening and (b) a second positioning opening. The first positioning opening i.e., positioning hole is arranged at a predetermined position in the plate-shaped workpiece to engage with a first positioning pin that is provided on the press die i.e., press mould. The second positioning opening i.e., positioning hole is arranged at a predetermined position that is spaced away from the first positioning opening, and is formed in an elongate shape to engage with a second positioning pin that is disposed in the press die. These first and second positioning openings operate to position the plate-shaped workpiece in a predetermined posture. Patent Document 1 discloses one example of a positioning structure that has an elongate second positioning opening accommodating thermal expansion and dimensional variations in the plate-shaped workpiece.
In addition to this, both the first and second positioning pins that engage with these openings are arranged to be able to protrude from and retract into the press die. The first and second positioning pins move to the protruded positions prior to the hot press forming in order to position the plate-shaped workpiece, and subsequently, prior to the hot press forming, they retract into their retracted positions. In this way, the first and second positioning pins allow the plate-shaped workpiece to be cooled and contractionally deformed with being kept in a press formed state, that is in a state tightly held by the press die, in the hot press forming.
Patent Document 1: Japanese Patent Publication No. 2006-224105.
However, in such known positioning structure, since the positioning pins retract during the hot press forming, the plate-shaped workpiece moves freely upon formation by the press die. Accordingly, depending on the forming shape, the plate-shaped workpiece may deviate when it is subjected to the press forming. In the hot press forming, the plate-shaped workpiece at high temperature is subjected to the press forming using the press die at low temperature to achieve forming and quenching at the same time. For this reason, the hot press forming is normally executed as a final process after trimming and so on were executed. Consequently, the aforementioned positional deviation of the plate-shaped workpiece may directly leads to defectiveness of the plate-shaped workpiece.
In addition to this, due to location of the aforementioned first positioning opening spaced away from a center of the plate-shaped workpiece, the position of the first positioning opening relative to an entire shape tends to deviate by thermal expansion and deformation of the plate-shaped workpiece caused with heating. Even when the position of the plate-shaped workpiece does not deviate during the press forming, high dimensional accuracy i.e., precision thereof can hardly achieved. In other words, the plate-shaped workpiece deforms symmetrically with respect to the center thereof during the thermal expansion, transformation and contraction due to cooling of the plate-shaped workpiece. If the first positioning opening is arranged at the position that is deviated from the center of the plate-shaped workpiece, expansion and deformation due to heat deteriorates i.e., degrades the positioning of the first positioning opening. As a result, positioning accuracy and forming accuracy of the plate-shaped workpiece deteriorate.
Furthermore, due to requirement of the protrusion/retraction mechanism for positioning pin, the positioning structure tends to be complicated and costly. In addition to this, these positioning pins are required to protrude/retract in every hot press forming process cycle, which lengthens each cycle time to restrict the forming efficiency, and requires maintenance for abrasion of movable parts to increase the forming costs.
The present invention has been developed in consideration of the above circumstances, and has the object to eliminate the requirement for the positioning pin to protrude/retract, to construct the forming device simply at low cost, and to restrict any positional deviation of the plate-shaped workpiece for achieving the high forming accuracy.